Incandescent lamps, fluorescent lamps, high-pressure discharge lamps and other types of lamps have been used as luminaries or light sources for billboards. Recently, an LED illumination source has been researched and developed as a new type of illumination source that could potentially replace these conventional light sources. An LED illumination source has a longer life than any of those conventional light sources, which is one of its advantageous features, and is widely expected to be a next-generation illumination source. To obtain a luminous flux comparable to that of an incandescent lamp or a fluorescent lamp, however, the LED illumination source needs to be an array of multiple LED elements because a single LED element has just a small luminous flux.
Hereinafter, conventional LED illumination sources will be described with reference to the accompanying drawings.
FIGS. 1(a) and 1(b) illustrate configurations of two conventional LED illumination sources. FIGS. 2(a) and 2(b) illustrate cross-sectional structures of LEDs included in the two types of LED illumination sources.
Each of these LED illumination sources includes a substrate 21 as shown in FIGS. 1(a) and 1(b). On the substrate 21, a number of LED bare chips 22 are mounted. As used herein, the “LED bare chip” refers to an LED that is yet to be mounted and yet to be molded with a resin, for example. For the sake of clarity, an LED that has already been molded so as not to expose its light emitting portion but still has not been mounted yet will be herein referred to as an “LED element”. On the substrate 21 shown in FIG. 1(a), a plate 23, including multiple holes 23a to transmit the light that has been emitted from the LED bare chips 22, is provided. On the other hand, on the substrate 21 shown in FIG. 1(b), a layered resin 24 that also transmits the light emitted from the LED bare chips 22 is provided. That is to say, the LED bare chips 22 are covered with the resin 24.
In each of these LED illumination sources, the LED bare chip 22 is mounted in a bare chip state on the substrate 21 as shown in FIGS. 2(a) and 2(b). The LED bare chip 22 includes a chip substrate 31 of sapphire, SiC, GaAs or GaP, and a light emitting portion that has been formed on the chip substrate 31. The light emitting portion is formed by stacking an n-type semiconductor layer 32 of GaN, for example, an active layer 33, and a p-type semiconductor layer 34 in this order. The electrode 32a of the n-type semiconductor layer 32 and the electrode 34a of the p-type semiconductor layer 34 are electrically connected to conductive lines 21a on the substrate 21 by way of Au wires 41 and 42, respectively. It should be noted that this configuration of the light emitting portion is just an illustrative one. Thus, the LED may have a quantum well, a Bragg reflector layer, or a resonant cavity structure.
In the configuration shown in FIGS. 1(a) and 2(a), the light that has been emitted from the LED bare chip 22 is reflected from a reflective plane 23a, which is the inner surface of a hole (or opening) 23b of the plate 23, and then goes out of the element. The hole 23b of the plate 23 is filled with the resin 24 so as to mold the LED bare chip 22 and the wires 41 and 42 together. On the other hand, in the configuration shown in FIGS. 1(b) and 2(b), the light that has been emitted from the LED bare chip 22 goes out of the element through the molding resin 24.
In the LED bare chip 22, when a forward bias voltage is applied between the electrodes 32a and 34a of the n- and p-type semiconductor layers 32 and 34, electrons and holes are injected into these semiconductor layers and recombine with each other. As a result of this recombination, light is created in, and emitted from, the active layer 33. In an LED illumination source, the light, emitted from multiple LED bare chips 22 that have been mounted on the substrate, is utilized as illumination.
The LED bare chip 22 generates a lot of heat when emitting the light. The heat generated is supposed to be dissipated from the substrate 21 by way of the chip substrate 31. However, to make such an LED illumination apparatus a commercially viable product, the following problems must be solved.
As described above, the luminous flux of each one of the LED bare chips 22 is small. Accordingly, to achieve desired brightness, quite a few LED bare chips 22 need to be arranged on the substrate 21. To avoid an excessive increase in size of the substrate even when a great number of LED bare chips 22 are arranged thereon, the LED bare chips 22 need to be mounted at an increased density.
Also, to increase the luminous flux of each LED bare chip 22 as much as possible, a current to be supplied to the LED bare chip 22 (e.g., an eddy current of about 40 mA with a current density of about 444.4 mA/mm2 per unit area, for example) needs to be greater than a current that is supplied for normal purposes other than illumination (e.g., about 20 mA with a current density of about 222.2 mA/mm2 per unit area for a 0.3 mm square LED bare chip, for example). However, when such a great amount of current is supplied to each LED bare chip 22, an increased quantity of heat is generated from the LED bare chip 22. As a result, the temperature of the LED bare chip 22 (which will be herein referred to as a “bare chip temperature”) rises to reach a rather high level. Generally speaking, the bare chip temperature has significant effects on the life of the LED bare chip. More specifically, it is said that when the bare chip temperature rises by 10° C., the life of an LED apparatus, including the LED bare chip 22, should be halved.
An LED is usually believed to have a long life. However, it is quite a different story if the LED is used for illumination purposes. What is worse, when the bare chip temperature rises with the increase in the quantity of heat generated, the luminous efficacy of the LED bare chip 22 decreases unintentionally.
In view of these considerations, to realize an LED illumination apparatus with a huge number of LED bare chips 22 mounted thereon at a high density, the heat dissipation performance should be improved, and the bare chip temperature should be decreased, compared to the conventional ones. The optical efficiency also needs to be increased to utilize the light, emitted from the LED bare chips 22, as illumination as efficiently as possible, or with the waste of the optical energy minimized.
To overcome these problems, various types of LED illumination sources with an array of LED bare chips thereon have been proposed. However, none of those conventional LED illumination sources has ever succeeded in coping with all of those problems satisfactorily.
Hereinafter, the problems of the conventional LED illumination sources will be described with reference to FIGS. 1(a), 1(b), 2(a) and 2(b). Firstly, if the LEDs are kept ON continuously for a long time, the center portion of the substrate, having the huge number of LEDs integrated thereon, gets more and more heated. As a result, the difference in temperature between the center and peripheral portions of the LED substrate escalates with time. The configuration shown in FIGS. 1(a) and 2(a) is adopted for an LED dot matrix display, for example. In an LED display, the plate 23 works in such a manner as to increase the contrast between the emitting and non-emitting portions of each LED. When LEDs are used for a display like this, not all of those LEDs are always kept ON at full power. Thus, not so serious a heat generation problem should happen in such a situation. However, when LEDs are used to make an illumination apparatus, all of those LEDs must be kept ON for a long time, thus causing a non-negligible heat generation problem.
In the conventional examples described above, the substrate 21 and the plate 23, both made of the same resin, are combined together, and have substantially the same thermal expansion coefficients. However, a resin material normally has a low thermal conductivity, and easily stores the heat generated. For that reason, such a resin material cannot be used so effectively in an illumination apparatus that should always be kept ON at the maximum output power.
Also, since there is a difference in temperature between the center and peripheral portions of the substrate 21 to be combined with the plate 23, a difference is also created between the thermal expansion coefficient of the center portion and that of the peripheral portion, thus imposing a great stress in the peripheral portion of the substrate. When LEDs are used in an illumination apparatus, a stress is caused by the heat every time the LEDs are turned ON and OFF repeatedly, which eventually leads to disconnection of the electrodes 32a or 34a of the LEDs.
The substrate itself may include a portion, which is as thick as the plate 23 and which is made of a material with a thermal conductivity that is approximately equal to the high thermal conductivity of the substrate material, instead of the plate 23 separately provided. Then, that substrate may include recesses to mount LED bare chips thereon. Even so, the heat-dissipating and uniform thermal distributing performance is also limited by the thermal conductivity of the substrate material.
Furthermore, when the above-described configuration is adopted, the substrate itself needs to be thick enough and the substrate to mount the LED bare chips 22 thereon cannot have a reduced thickness. For that reason, even if the substrate material has a high thermal conductivity, the heat is still stored easily in the substrate. Accordingly, when kept energized or ON for a long time with a great amount of current supplied as in an illumination apparatus, the LED bare chips, mounted around the center of the substrate, in particular, will have noticeably increased temperatures, thus creating a big temperature difference between the center and peripheral portions of the substrate. Consequently, the properties of the substrate material with the high thermal conductivity cannot be made full use of, and the heat dissipation problem is still insoluble. Furthermore, unless the recesses to be provided on the surface of the substrate have relatively large sizes, a sufficient space cannot be allowed to mount the LED bare chips 22 thereon and conductive line the LED bare chips 22 by the wire bonding technique. In addition, the optical system used should have an increased size. Also, it is currently difficult to mount the LED bare chips 22 inside the recesses considering the capillary and collet sizes of bonders of various types. That is to say, to get the capillary or collet inserted into the recess, the recess and the optical system (i.e., the light outgoing region) should have increased sizes.
In the configuration shown in FIGS. 1(b) and 2(b) on the other hand, one side of the substrate 21 is covered with the molding resin 24. Accordingly, the time it takes to cure the molding resin 24 in the center portion of the substrate is different from the time it takes to cure the same resin 24 in the peripheral portion thereof. As a result, a great residual stress is produced inside the resin. Furthermore, since the light that has been emitted from one LED bare chip 22 is absorbed into other LED bare chips 22 (i.e., self-absorption by LEDs), the light-extraction efficiency of the overall LED illumination source decreases. Moreover, since the molding resin 24 functions as a heat storage material, a temperature difference is created between the center and peripheral portions of the substrate. In that case, the thermal expansion coefficient also varies on the same substrate, thus propagating the stress of the molding resin 24 to the peripheral portion of the substrate.
In order to overcome the problems described above, an object of the present invention is to provide an LED illumination source and LED illumination apparatus that can solve all of these problems (i.e., increasing the density of integration and improving the heat dissipation performance and optical efficiency) at the same time.